A Basic Model for Star Birth

I include a sequence of pictures of an imaginary cross section through the cloud to illustrate how we can imagine a star being born.

Start with cool gas.
- Starting picture, part of a giant molecular cloud, cool and dense.
- If it's too hot, the gas pressure will be to high and nothing will happen.
- So keep away from hot stars.
- A little dust to block the starlight will help.
The cool gas is attracted to the center by gravity.
- Recall that every atom attracts every other atom.
- The closer the atoms are, the bigger the attraction.
The cloud gets smaller and denser.
- The cloud starts to contract. The cloud fragment is about 10⁴ AU in size.
- Smaller scale. The cloud continues to pull itself together
It starts to rotate because of conservation of angular momentum.
- The easiest way to see this is to demonstrate it. It's like a figure scater doing a spin.
The outside parts form into a disk.
- A disk is the only possible shape for gas in orbit around a central mass.
- The disk configuration keeps gas cloudlets from hitting each other.
- The disk is called a protoplanetary disk.
- Still smaller scale. The cloud is smaller and is rotating.
As the central part of the disk continues to contract, it gets warmer.
- Whenever you compress a gas, it gets warm.
- This is energy conservation at work: gravitational potential energy is turned into heat energy.
As the temperature goes up to several thousand K, the protostar starts to radiate.
Still smaller scale. A protostar has condensed in the middle. The protostar is about 1 AU in size; the whole picture is about 100 AU in size
The inside continues to heat up as it contracts.
When the center gets dense enough and hot enough ( a few x 10⁶K), nuclear fusion begins.
The central heat source stabilizes the pressure, halting further contraction.
- Now the energy radiated is supplied by nuclear fusion instead of further contraction.
A star has been born.

Davison E. Soper, Institute of Theoretical Science, University of Oregon, Eugene OR 97403 USA soper@bovine.uoregon.edu